Performance considerations

You’ve likely wondered why the log/slog package has some odd-looking functions and concepts in some places. Why do you set a handler’s level to a Leveler value, rather than a simple Level? Why so many ways to create key/value pairs ("key", "value" vs "key", slog.AnyValue("value") vs "key", slog.StringValue("value") vs slog.Any("key", "value") vs slog.String("key", "value"))?

It mostly comes down to one thing: Performance.

Or, more accurately, trying to balance performance with an easy-to-use API. These two goals are somewhat at odds. And that’s why we have more than one way to do many things in this package—the “easy” way, and the “performant” way.

Most often, you don’t need to care. Most parts of most apps are not performance critical.

But some parts of some apps ARE! And the next few days we’ll be coverting the doc section for those of us working on such problems.

Performance considerations

If profiling your application demonstrates that logging is taking significant time, the following suggestions may help.

If many log lines have a common attribute, use Logger.With to create a Logger with that attribute. The built-in handlers will format that attribute only once, at the call to Logger.With. The Handler interface is designed to allow that optimization, and a well-written Handler should take advantage of it.

This first hint is actually a happy place where the high-performance option overlaps with the easier-to-use case as well:

log.Info("starting request", "url", fullURL)
/* ... */
log.Info("request served", "url", fullURL)

Can be simplified:

log = log.With("url", fullURL)
log.Info("starting request")
/* ... */
log.Info("request served")

The benefit of log.With compounds the more keys you have, and the more times you need common log attributes.

I’ll typically use log.With when I construct my logger, to inject some common attributes into every log the application produces:

log := slog.New(...)
log = log.With(
  "app",        appName,
  "version",    appVersion,
  "git_sha",    gitSHA,
  "build_time", buildTime,
  "server_ip",  serverIP,
  /* etc, etc */
)

Performance considerations

May 22, 2026